In metal forming processes, metal melt is transferred from one metallurgical vessel to another, to a mould or to a tool. For example, a tundish of large capacity is regularly fed with metal melt by a ladle transferring metal melt from a furnace to the tundish. This allows the continuous casting of metal from the tundish to a tool or mould. Flow of metal melt out of metallurgic vessels is driven by gravity through nozzle systems located at the bottom of the vessels, usually provided with a gate system to control (open or close) the flow of metal melt through said nozzle system. In order to resist the high temperatures of metal melts, the walls of the vessels are lined with refractory material.
Metal melts, in particular steel, are highly reactive to oxidation and must therefore be sheltered from any source of oxidative species. Small amounts of aluminum are often added to passivate the iron in case oxidative species enter into contact with the melt. In practice, it appears that often this is not enough to prevent the formation of oxide inclusions in the melt that produce defects in a final part produced from the melt. It is reported that a 10 kg steel casting may contain up to one billion non-metallic inclusions, most of them being oxides. Inclusions must be removed from the final part by grinding or cutting; these procedures add to the production cost and generate large amounts of scrap.
Inclusions may be the result of reactions with the metal melt; these inclusions are known as endogenous inclusions. Exogenous inclusions are those in which materials not resulting from the metal melt, such as sand, slag, and debris of nozzles; exogenous inclusions are generally thicker than endogenous inclusions.
Endogenous inclusions comprise mostly oxides of iron (FeO), aluminium (Al2O3), and of other compounds present in, or in contact with the melt, such as MnO, Cr2O3, SiO2, TiO2. Other inclusions may comprise sulfides and, to a minor extent, nitrides and phosphides. Since metal melts are at very high temperatures (of the order of 1600° C. for low carbon steels) it is clear that the reactivity of an iron atom with an oxide is very high and reaction cannot be prevented.
To date, most measures to reduce the presence of inclusions in a steel casting is to retain them in the metallurgical vessel in which they were formed. The present invention proposes a radically different solution by reducing substantially the formation of endogenous inclusions in a metallurgical vessel with simple, reliable, and economical means.